1. Field of the Invention
The present invention relates to an apparatus for generating a bias current having an adjustable temperature coefficient, and more particularly, to a bias current generating apparatus capable of providing a bias current where a characteristic change is compensated, to one of an analog circuit and radio frequency (RF) circuit where various characteristic changes occur according to a temperature, by generating various bias currents having a plurality of temperature coefficients.
2. Description of the Related Art
In general, it is essential to provide performance of a circuit and an yield that stability with respect to a temperature, power voltage, and changes in process is provided in designing a complementary metal-oxide semiconductor (CMOS) analog or radio frequency (RF) circuit. Particularly, in such a stable circuit design, since a bias circuit is an element having a direct effect on performance of analog or RF circuit, it is very importance to provide stable operations of the bias circuit.
However, a transistor forming the analog or RF circuit has properties varying with a change in a temperature, there is required a bias circuit capable of stably complementing the change. A threshold voltage and mobility are greatest property of the transistor, which varies with a temperature change. Due to changes in the threshold voltage and mobility, transconductance of a metal-oxide semiconductor (MOS) transistor is changed. Generally, since transconductance decreases as a temperature is increased, a bias circuit capable of complementing the properties.
In general, as a method of complementing characteristic changes with respect to a temperature in designing a CMOS circuit, there is well-known technology of using a band gap reference circuit generating a stable bias current or voltage. A positive temperature coefficient (i.e. Proportional to Absolute Temperature: PTAT) circuit included in such band gap reference circuit has a positive temperature coefficient with respect to an absolute temperature. Accordingly, as a temperature is increased, a bias current or voltage increases. Applying the positive temperature coefficient circuit, it is possible to compensate a temperature in a limited range.
However, since a positive temperature coefficient circuit employed by a conventional band gap reference circuit has a fixed temperature coefficient, there is a limitation on being applied to a circuit having various variance properties. That is, analog or RF circuits include not only an MOS transistor but also a passive device such as a resistor. Such passive device also requires a temperature compensation circuit capable of varying a temperature coefficient. Also, since various circuits have different temperature coefficients of bias circuits, required for temperature compensation, there is required a temperature compensation circuit having various temperature coefficients.
As described above, in the art, there is required a temperature compensation bias circuit capable of providing a bias current where the characteristic changes are compensated, to various circuits having different characteristic changes according to a temperature.